1. Field of the Invention
The present invention relates to an image forming apparatus having an improved home position sensing structure of a transfer belt, and a method of disposing a home position sensing apparatus of the transfer belt for the image forming apparatus.
2. Description of the Related Art
Generally, a color image forming apparatus using an electro-photographic method forms an electrostatic latent image by scanning light onto a photosensitive medium charged at a predetermined electric potential. The electrostatic latent image is developed by using predetermined toner of a development unit. The electrostatic latent image is transferred onto a recording medium by a transfer unit. A color image is then formed by a settlement unit. Color toners for the color image forming apparatus are usually yellow (Y), magenta (M), cyan (C) and black (K), and four development units are required to form the four color toner, respectively.
FIG. 1 shows a structure of a photosensitive medium and a transfer unit of a conventional image forming apparatus.
With reference to FIG. 1, a photosensitive drum 1 is employed as the photosensitive medium, and a transfer belt 3 is employed as the transfer unit. Reference numerals 5A-5E in FIG. 1 denote rotation rollers supporting the transfer belt, which is driven in a caterpillar-type manner.
The electrostatic latent image formed on the photosensitive drum 1 is reproduced as a toner image by a developing device (not shown), and each color toner image is sequentially overlapped on the transfer belt 3 to form a color toner image. This transferring process is important since it influences the quality of printing. Thus, when sequentially transferring the toner image formed on the photosensitive drum 1 onto the transfer belt 3 by each color, the image should be transferred without misaligning each color. Herein, it is required to accurately synchronize the point in time of forming the electrostatic latent image on the photosensitive drum 1 with that on the transfer belt 3.
In the conventional art, a sensor 10 senses a home position of the transfer belt, and then a color printing initial position is determined by using the home position as a standard for forming the electrostatic latent image.
FIG. 2 is a sectional view taken along the line II-II′ of FIG. 1, showing a structure of a sensor sensing a home position of a transfer belt.
With reference to FIG. 2, a light penetrating portion 5 is provided at one side of one end of the transfer belt 3, and protection tapes 7a and 7b are adhered to both sides of both ends of the transfer belt 3. A guide member 9 is positioned on the protection tape 7b attached to the other surface of the transfer belt 3 having a transfer surface 3a. The guide member 9 guides the transfer belt 3 to be regularly driven in the caterpillar-type manner without breaking away from the track and is slidedly inserted in guide grooves 5A′-5E′ (referring to FIG. 1) formed at both ends of the rotation rollers 5A-5E. An external surface of the transfer belt 3, that is, the surface in contact with the photosensitive drum 1, is the transfer surface 3a. 
The sensor 10 includes a sensor body 15 having a first sensing element 11 (a light emitting portion) and a second sensing element 13 (a light receiving portion) that are positioned to be spaced from both sides of the transfer belt 3, respectively. The sensor 10 in the aforementioned structure emits a predetermined light from the light emitting portion 11 and senses a home position of the transfer belt 3 according to whether the light receiving portion 13 receives the light. That is, when the light penetrating portion 5 of the transfer belt 3 driven in the caterpillar-type manner is positioned in the path through which the light passes, the light emitted from the light emitting portion 11 penetrates the light penetrating portion 5 and reaches the light receiving portion 13. Sensing such a state, the home position of the transfer belt 3 is sensed.
However, in the conventional sensing structure described above, there is a problem in that a distance between the transfer surface 3a of the transfer belt 3 driven in the caterpillar-type manner and the light receiving portion 13 is short. In this case, if the transfer belt 3 droops due to a length change, a tension change or tension inequality at both ends of the transfer belt 3 resulting from the breakaway of the transfer belt 3 from the track, the transfer surface 3a interferes with the light receiving portion 13. Consequently, the interference with the light receiving portion 13 causes sensing errors.
Furthermore, if scattering toner is stuck, this also becomes a factor of contaminating the light receiving portion 13, thereby causing sensing errors.
Accordingly, a need exists for an image forming apparatus having an improved sensing device in which the transfer belt does not interfere with the sensing element.